I have no difficulty with them re-casting Simm and the nature of the Master's role is that, like the Doctor, he can be reinvented. Having said that, it's been a long time (if ever) since any actor played the Master opposite two different Doctors (I believe Anthony Ainsley may have done so, no?) so it might be interesting to see how Simm's Master might react to a new Doctor, having only ever encountered Ten.